The present invention relates to head up display systems and in particular to a head up display system wherein flare or multiple images are minimized and a substantially aberration-free display is presented.
A head up display for an aircraft conventionally includes a "combiner" through which the aircraft pilot views the outside world and through which the pilot can also view instrument information or symbology useful in flying the aircraft. Thus, the pilot need not lower or refocus his eyes to see certain instrument readings but can view the exterior scene while vital instrument information is projected in his line of sight. Several types of head up displays are well known including conventional systems using a half-silvered mirror or dielectric reflector combiner through which the pilot looks and upon which symbology is projected for viewing. Unfortunately, this type of head up display is very inefficient and tends to reduce visibility of the outside scene. Also the symbology may be obscured by bright exterior light, and the field of view is restricted.
A more satisfactory head up display employs a holographic optical element as a combiner. This type of combiner has advantages of excellent light transmission of the exterior scene and wavelength selective reflection of symbology projected thereupon. As a result, the exterior view is not obscured and the symbology presented upon the combiner is bright and easily seen even under high ambient light conditions. A disadvantage characteristic of holographic head up display relates to "flare" or multiple imaging. Thus, if a bright outside source is viewed through the combiner, such as runaway lights at night, the appearance of flare or an undesired brightness around the object is perceived. If a bright extended object is viewed, multiple images appear around the bright object.
In the copending application of Robert B. Wood and Robert F. Cannata Ser. No. 349,924, filed Feb. 18, 1982, for "HOLOGRAPHIC DEVICE", a flare-free system is described wherein a holographic element used in a combiner has a low surface spatial frequency, i.e. with few fringes intersecting the hologram surfaces. The undesirable effect of these fringes is thereby substantially eliminated resulting in essentially a flare-free system. The term "flare" in the present application is used to describe the spectral and spatial dispersions of real world objects seen through a combiner caused by the undesired transmission grating behavior of a reflecton holographic element having a surface spatial frequency, e.g. when fringes intersect the hologram surface. The surface spatial frequency is reduced by controlling the angular relationship of the exposure rays when forming the hologram, and in a particular case by maintaining the exposure beam from a point source substantially perpendicular to the surfaces of the gelatin or other sensitized material used in constructing the hologram. The holographic combiner formed in the above manner is highly advantageous, but is somewhat restricted in optical power.
In conventional holographic head up displays, a "relay lens" system is used to project the desired symbology toward the holographic combiner at an angle relative to the combiner, with the relay lens system being provided with aberration correction features compensating for the aberrations resulting from the off-axis projection angle. At small or moderate off-axis angles, conventional relay optics are capable of providing a preaberrated image, which, when viewed directly through the combiner, will appear to be in correct proportion. However, at large off-axis angles in a wide field of view system it is difficult to compensate for the aberrations produced by the combiner without also providing aberration correction optical features in the combiner itself. A large off-axis angle results when the holographic projection system is located largely below the windscreen and under the instrument panel in crowded aircraft cockpit constructions. And even though aberration correction power can be built into the combiner as in the prior art, the combiner is then no longer flare-free.